psy 60 winter 2010 ch2 outline - Chapter 2 Describing...

Chapter 2 Describing Distributions with Numbers Measuring center: the mean – “average” value  Sigma – “add them all up”  Not resistant: a few outliers can make s very large. Measuring center: the median – “typical” value  Median – midpoint of a distribution  Resistant: outliers do not matter median  If # of observations n is odd – count (n+1)/2 observations from any sides  If # of observations n is even – count (n+1)/2 observations then plus then divide by 2 The mean and median of a roughly symmetric distribution are close together. If the distribution is exactly symmetric, the mean and median are exactly the same. In a skewed distribution, the mean is usually father out in the long tail than is the median. Measuring spread: the quartiles  First quartile – median of the n observations left to the median, larger than 25% of all obs.  Second quartile – median (larger than 50% of observations)  Third quartile – median of the n observations right to the median, larger than 75% of all obs.  Quartiles are resistant

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